Photogrammetry is a non-contact method of shape measurement that involves taking multiple two-dimensional images of a three-dimensional object from a plurality of different angles, and triangulating the two-dimensional images to produce a three-dimensional surface model of the object. When manufactured parts require assembly, the precise shape and size of the parts is required. This is especially true of delicate, complex, large and expensive aircraft, watercraft and/or automotive components where the accuracy of part shape is determined prior to attempted assembly. Photogrammetry is useful for testing, modeling, reproducing and measuring surface variations of an object. Parts that are tested with photogrammetric processes often require fewer jigs and other fixtures during assembly, thereby reducing production costs and assembly time.
Two-dimensional images used in photogrammetry contain a series of targets that represent co-ordinate areas on a three-dimensional object. Each target represents a precise location on a three-dimensional surface and can be used to identify common locations among multiple two-dimensional images. Targets are either manually applied or projected onto the three-dimensional object. One of the most commonly used methods of applying targets to an object is to project dots of white light onto the surface of an object. Typically, a software package is used that will outline the three-dimensional object and record the projected targets. Every target in every image must be identified and cross-referenced with other targets representing the same area in other images. Photogrammetric processes require that each target appear in at least two two-dimensional images for cross-referencing purposes. Once the targets are cross-referenced, the software orients each image by aligning identical targets in multiple images. This process of triangulation allows a plurality of two-dimensional images to model a three-dimensional object.
One of the constraints of photogrammetry is that the coordinates of an object are determined only at the target locations. To obtain as accurate a surface contour as possible, it is desirable to project a dense array of targets onto the surface of the object. When an object surface has steep contours, a particularly dense array of targets must be used to capture the change in surface elevation. One of the significant problems associated with traditional methods of photogrammetry is that a crowded array of targets can lead to ambiguity in the identification of the individual targets within the plurality of two-dimensional images. The denser the target arrangement becomes, the more difficult it is for a software program to identify a particular target in a plurality of images. In these circumstances, an operator must manually identify “reference” targets in each of the images to assist the software in identifying the remaining targets. This is a time consuming process, and often requires multiple attempts before the target pattern is successfully identified and indexed. Accordingly, a need remains for an improved method and apparatus for identifying specific targets within a target array for photogrammetric processes.
The present invention has been developed in view of the foregoing.